1. Increase the font-size from 13px to 16px and set line-height 24px to make the type friendly.
2. Increase the height of your input fields and buttons to 28px to make them easier to focus and click on.
3. For the diary image on your front page, show a diary that has something written in it. Even better, show a screenshot of a Duuble diary.
4. Combine "First name" and "Last name" fields into "Your Name" for better internationalization and less visual elements / tabbing.
5. Move the "Tweet" button away from your sign up form to top right of the page. Use the official Twitter button. People are used to it.
6. The "Read more about Duuble" button should be a link not a button.